32 



PLANT HABITS AND HABITATS IN THE 



is accompanied by a change in the ''relative transpiration," or tran- 

 spiration power (relation between rate of transpiration and rate of 

 evaporation) . Under dry conditions the t/e ratio is greater by day, but 

 under moist conditions it is greater by night (Shreve, E. B., 1915: 79). 

 The fluids of desert plants have a high concentration, as determined by 

 Fitting (1911:209), Lawrence, Gortner, and Harris (1916:1). The 

 concentration of the juices varies in relation to local environmental 

 conditions. It is least in the aiToyos and greatest in the salt spots. 

 For example, an average of eight determinations of the density of the 

 juices of plants from the latter habitat gave 37.1 atmospheres. Table 

 9 summarizes these results. 



Table 9. — Osmotic pressure, in atmospheres, of various growth-forms in five habitats of tht 

 Tucson region {Harris, 1916 : 81). 



Richards (1918: 64) finds that a certain species is more or less suc- 

 culent when growing under dry conditions, whereas the typical forms, 

 under moist conditions, have thin leaves. In every instance the more 

 succulent form developed less acid than the form less succulent. 



The dryness of the atmosphere works immediately to influence the 

 formation of a greater amount of cellulose and a lesser amount of 

 starch (MacDougal and Spoehr, 1918^:247). Thus the polysacchar- 

 ides are converted into anhydrides or wall material under conditions 

 of aridity, or in succulent species, polysaccharides are converted into 

 pentosans or mucilages. These changes, particularly the last, are of 

 great physiological importance to the species, inasmuch as the "imbi- 

 bition " capacity of the polysaccharides is small. Their transformation 

 from this form into that of the pentosans gives the increased capacity 

 (of imbibition) characteristic of the pentosans, so that without any 

 addition of material to a cell, but simply by the loss of water, a change 

 takes place by which the cell is capable of absorbing and holding vastly 

 greater proportions of water. 



Low water-content of certain cacti results in a condition of general 

 reversion of carbohydrates to polysaccharides. The simpler sugars, or 

 monosaccharides, decrease in amount in the plants as the water-content 

 is reduced. With continued low water-content the pentosans increase 

 decidedly (Spoehr, 1918:62). 



It would appear, therefore, that dryness of itself may profoundly 

 modify the chemical nature of plants exposed to its influence and it may 

 lead, as indicated above, on the one hand to formation of wall material, 



